1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus wherein radiation images of the mamma, on which energy subtraction processing is to be carried out, are recorded and read out. This invention particularly relates to a radiation image recording and read-out apparatus wherein two radiation images of the mamma are sequentially stored on a stimulable phosphor sheet, the stimulable phosphor sheet, on which each radiation image has been stored is exposed to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, the emitted light being detected and converted into an electric image signal representing the whole radiation image, two electric image signals thus detected are then subtracted from each other, and a subtraction image is thereby obtained.
2. Description of the Prior Art
Techniques for carrying out energy subtraction processing on radiation images have heretofore been known. In energy subtraction processing, such characteristics are utilized that a specific structure of an object (for example, the organs, bones, or blood vessels in cases where the object is a human body) exhibits inherent radiation energy absorption characteristics. Specifically, an object is exposed to at least two kinds of radiation having different quality, and two radiation images, in which different images of a specific structure are embedded, are obtained. Thereafter, the image signals representing the two radiation images are weighted appropriately, and subjected to a subtraction process in order to extract the image of the specific structure.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed to several kinds of radiation having different energy distributions, or the energy distribution of the radiation, which has passed through an object, is changed after it has been irradiated onto one of two radiation storage means, after which the radiation impinges upon the second storage means. In this manner, two radiation images, in which different images of a specific structure are embedded, are obtained. Thereafter, the image signals representing the two radiation images are weighted appropriately, when necessary, and subjected to a subtraction process in order to extract the image of the specific structure.
Subtraction processing is extremely effective, particularly for medical diagnosis, and electronics research has continued to develop improved subtraction processing methods.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which has passed through an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed as desired, and the processed image signal is then used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device. In the radiation image recording and reproducing systems, the stimulable phosphor sheet is used to store the radiation image temporarily so that a final visible image can be reproduced therefrom on a final recording medium. For the sake of economy, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Also, a mobile X-ray diagnostic station, e.g. a bus serving as such may be provided with a radiation image recording and read-out apparatus wherein stimulable phosphor sheets are utilized. In cases where such a mobile X-ray diagnostic station moved from place to place so that mass medical examinations may be carried out, it is disadvantageous to have to load the mobile X-ray diagnostic station with a large number of stimulable phosphor sheets, and therefore the number of stimulable phosphor sheets which can be carried along with the mobile X-ray diagnostic station is limited. Therefore, it is desirable that a small number of reusable stimulable phosphor sheets be carried along with the mobile X-ray diagnostic station, and that the stimulable phosphor sheets be circulated and reused for the recording and readout of radiation images. Image signals detected from the stimulable phosphor sheets may be stored on a storage medium having a large storage capacity, such as a magnetic tape. With such a mobile X-ray diagnostic station, image signals representing the radiation images of many objects can be obtained. Also, if the stimulable phosphor sheets are circulated and reused, the recording of radiation images can be carried out continuously, and it becomes possible to increase the speed, with which radiation images are recorded in mass medical examinations. This is very advantageous from the point of view of practical use.
In order that the stimulable phosphor sheets may be reused in the manner described above, any energy remaining on the stimulable phosphor sheets after they have been scanned with stimulating rays during the readout of radiation images therefrom should be erased. For this purpose, the method disclosed in, for example, U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599 may be employed. The erased stimulable phosphor sheets can then be reused to record radiation images.
Therefore, it is desirable that there be combined into a single apparatus: an image recording section in which a stimulable phosphor sheet capable of being circulated and reused is exposed to radiation, which has passed through an object, an image read-out section in which a radiation image stored on the stimulable phosphor sheet is read out, and an erasing section in which any energy remaining o the stimulable phosphor sheet after the radiation image has been read out therefrom is erased. A mobile X-ray diagnostic station allows medical examinations to be carried out at different locations, and the aforesaid radiation image recording and read-out apparatus is easy to load into such a mobile X-ray diagnostic station. Also, such an apparatus can easily be located in a hospital, or the like. This is very advantageous for practical use.
In the aforesaid radiation image recording and reproducing systems utilizing a stimulable phosphor sheet, the radiation image stored on the stimulable phosphor sheet is read out directly as an electric image signal. Therefore, with such radiation image recording and reproducing systems, the aforesaid subtraction processing can readily be carried out. In cases where energy subtraction processing is to be carried out, radiation images may be stored on two stimulable phosphor sheets so that the parts of the radiation images corresponding to a specific structure are different in the two radiation images. For this purposes, two-shot energy subtraction processing may be employed wherein the operation for recording a radiation image is carried out twice with two kinds of radiation having different energy distributions. Alternatively, one-shot energy subtraction processing may be employed wherein, for example, two stimulable phosphor sheets placed one upon the other are simultaneously exposed to radiation, which has passed through an object, such that they are exposed to radiation having different energy distributions.
In order to carry out one-shot energy subtraction processing, the following methods have been proposed:
(1) A method wherein a filter, which is constituted of a metal or the like and which absorbs low energy components of radiation, is located between two stimulable phosphor sheets, and radiation having different energy distributions is thereby obtained. PA0 (2) A method wherein two stimulable phosphor sheets provided with layers of different types of stimulable phosphors are utilized so that no filter need be used and radiation images to be subjected to subtraction processing can be recorded with a single image recording operation. With this method, a stimulable phosphor sheet provided with a stimulable phosphor layer, which absorbs more of the low energy components of the radiation than the stimulable phosphor layer of the other stimulable phosphor sheet, is positioned closer to the object (closer to the radiation source), and the two stimulable phosphor sheets are simultaneously exposed to radiation. Such a method is disclosed in, for example, U.S. Pat. No. 4,855,598. PA0 i) a conveyance means for conveying a single stimulable phosphor sheet, which is capable of storing a radiation image thereon, along a predetermined path, PA0 ii) an image recording section which is located in the vicinity of said path and which is provided with: PA0 iii) an image read out section which is located in said path and which is provided with: PA0 iv) an erasing section which is located in said path and in which, before a next radiation image is stored on said stimulable phosphor sheet after said image signal has been obtained therefrom in said image read-out section, energy remaining on said stimulable phosphor sheet is erased, PA0 v) a sheet moving means for moving said stimulable phosphor sheet from said image recording section to said image read-out section and then to said erasing section along said path, said sheet moving means thereafter moving said stimulable phosphor sheet to said image recording section, PA0 vi) a radiation change-over means with which the quality of the radiation in said image recording section is changed over between at least two kinds of quality, PA0 vii) a control means which operates said sheet moving means in synchronization with the change-over operation of said radiation change-over means, such that two radiation images are sequentially stored with the radiation having different quality on said stimulable phosphor sheet during two image recording operations, and said two radiation images stored with the radiation having different quality during two image recording operations are read out during two image read-out operations in said image read-out section, and PA0 viii) an operating section in which image signals, which represent the two radiation images stored with the radiation having different quality and which were detected during the two image read-out operations in said image read-out section, are weighted and then subtracted from each other.
In cases where a radiation image of the mamma is recorded, a cassette in which a stimulable phosphor sheet is housed is located under the mamma. Therefore, if two-shot energy subtraction processing is employed, the mamma moves when the cassette is exchanged with a new one during the two image recording operations. As a result, an artifact occurs in a subtraction image obtained from the subtracting operations. For this reason, two-shot energy subtraction processing cannot be employed for mamma images. Accordingly, in order for energy subtraction processing for mamma images to be carried out, the one-shot process, wherein two radiation images are recorded with a single, simultaneous exposure to radiation, must have heretofore been used.
With one-shot energy subtraction processing, two radiation images can be recorded simultaneously, and therefore no artifact due to movement of the object (mamma) occurs in the subtraction image. However, oneshot energy subtraction processing has the drawback in that the image quality of the radiation image stored on the second stimulable phosphor sheet is lower than the image quality of the radiation image stored on the first stimulable phosphor sheet.
Therefore, it is desired that energy subtraction processing for mamma images can be carried out with the two-shot process, which yields a subtraction image having good image quality, such that no artifact due to movement of the object occurs in the subtraction image.
A novel apparatus for carrying out an energy subtraction processing method using stimulable phosphor sheets has been disclosed in, for example, U.S. Pat. No. 4,859,849. With the disclosed apparatus, at least two stimulable phosphor sheets are circulated through an image read-out section and an erasing section. However, such an apparatus is of an excessively large scale and unsuitable for the recording of mamma images from the point of view of the cost and floor space requirement. Therefore, a need exists for an apparatus which is small and which can yield an energy subtraction image of the mamma having good image quality.